# Black holes: The ultimate quantum computers?

Nearly all of the information that falls into a black hole escapes back out, a controversial new study argues. The work suggests that black holes could one day be used as incredibly accurate quantum computers – if enormous theoretical and practical hurdles can first be overcome.

Black holes are thought to destroy anything that crosses a point of no return around them called an “event horizon”. But in the 1970s, Stephen Hawking used quantum mechanics to show black holes do emit radiation, which eventually evaporates them away completely.

Originally, he argued that this “Hawking radiation” is so random that it could carry no information out about what had fallen into the black hole. But this conflicted with quantum mechanics, which states that quantum information can never be lost. Eventually, Hawking changed his mind and in 2004 famously conceded a bet, admitting that black holes do not destroy information.

But the issue is far from settled, says Daniel Gottesman of the Perimeter Institute in Waterloo, Canada. “Hawking has changed his mind, but a lot of other people haven’t,” he told **New Scientist**. “There are still a lot of questions about what’s really going on.”

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## Quantum entanglement

Now, Seth Lloyd of the Massachusetts Institute of Technology in the US, has used a controversial quantum model called final-state projection to try to solve the paradox. The model holds that under certain extreme circumstances – such as the intense gravitational field of a black hole, objects that would ordinarily have several options for their behaviour have only one. For example, a black hole could cause a coin thrown into it to always come up “heads”.

This allows information to escape from a black hole without any ambiguity about how to interpret it. The information escapes through a quantum process called entanglement, in which objects are not independent if they have interacted with each other or come into being through the same process. They become linked, or entangled, such that changing one invariably affects the other, no matter how far apart they are.

In black holes, Hawking radiation arises just inside the event horizon and has two components – one that leaves the black hole and another that falls towards the point-like singularity that is the black hole itself.

These components are entangled, so when matter that has been sucked into the black hole interacts with the infalling Hawking radiation at the singularity, the interaction instantaneously produces a change in the Hawking radiation that has escaped the black hole. Because the final-state projection model forces this interaction to behave in only one way, this radiation therefore carries information about material inside the black hole.

## Smooshed up

Gottesman and colleague John Preskill of the California Institute of Technology in Pasadena, US, found that previous calculations by other researchers using this model allowed information to escape for only certain interactions between the infalling matter and the infalling Hawking radiation. Now, Lloyd has calculated that the process is quite robust – the random nature of these interactions means the system is almost perfectly entangled.

That suggests the outgoing Hawking radiation carries away nearly all of the information of the matter – such as a spaceship – that falls into the black hole. According to Lloyd, the most that could be lost is half a quantum unit of information, or 0.5 qubit.

“Passengers on a spaceship would like some guarantee that when they fall into this black hole and get smooshed into the singularity, they can be recreated as it evaporates,” Lloyd told **New Scientist**. “With a few simple precautions, the travellers would be almost exactly the same, with less than an atom of difference.”

Lloyd also says the work suggests black holes could be used as quantum computers. “We might be able to figure out a way to essentially program the black hole by putting in the right collection of matter,” he says.

## Mission implausible

But both applications would require an understanding of the properties of specific black holes, says Gottesman. “And you’d have to collect every little piece of Hawking radiation because the spaceship would get spread out with everything that fell into the black hole – ever,” Gottesman says. “So you’d have to sort out which bits were the spaceship and which bits were other things. It’s implausible.”

Lloyd agrees. Understanding how to decode the outgoing Hawking radiation will require researchers to weave together quantum physics and general relativity into a seamless theory of quantum gravity – a goal that has so far proved elusive. “Until we understand quantum gravity, we’re not going to be running Linux on a black hole,” he jokes.

But beyond the practical difficulties, Gottesman says the work has a more serious theoretical flaw. Despite the fact that just half a qubit of information is lost, “from a fundamental point of view, there is no real difference between a little bit of information being lost and a lot being lost,” he says.

“In standard quantum mechanics, no information is ever lost, so if he is right, quantum mechanics would have to be revised to allow information loss. We have no real idea of what theory could take its place.”

Journal reference: *Physical Review Letters* (vol 96, no 061302)

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